For example, in the case of air conditioners and the like, while the power supply voltage of the outdoor unit is three-phase AC voltage of 200 V, the power supply of a communication system in the indoor unit and the like may require DC voltage of 60V. In this case, the three-phase AC voltage of 200V needs to be converted to DC voltage of 60V for supply to the communication system.
FIG. 24 is a circuit diagram of a conventional power converter. In this power converter, AC voltage supplied from an AC power supply S through first and second input connections T1 and T2 is stepped down and half-wave rectified to DC voltage, and then supplied to loads (not shown) through first and second output connections T3 and T4.
Between the first and second input connections T1 and T2, a step-down resistance unit RU including a plurality of resistors, a diode D11 , and a capacitor C11 are interposed in series in the order described from the first input connection T1 side. The forward direction of the diode D11 is from the first input connection T1 side toward the second input connection T2. Further, a plurality of series-connected Zener diodes ZD11 to ZD13 are connected in parallel to the capacitor C11. The forward direction of these Zener diodes ZD11 to ZD13 is from the second input connection T2 side toward the first input connection T1. Further, a resistor R11 for discharge of the capacitor C11 is connected in parallel to the capacitor 11.
The first output connection T3 is connected to a connection on the downstream side in the forward direction of the diode D11, and the second output connection T4 is connected to the second input connection T2.
More specifically, for example, an AC voltage of 200 V (peak value) is applied from the AC power supply S to the first input connection T1 with reference to the potential of the second input connection T2. Then, this voltage is converted into a DC voltage of 60 V. Corresponding to this, the resistance unit RU employed has a resistance value necessary to step down AC voltage of 200 V to DC voltage of 60 V. The capacitor C11 has a capacitance of 470 μF, and the Zener diodes ZD11 to ZD13 each have a Zener voltage of 20 V.
Then, the AC voltage supplied from the AC power supply S is stepped down by the resistance unit RU, half-wave rectified by the diode D11, stabilized by the capacitor C11 and the Zener diodes ZD11 to ZD13, and outputted as a DC voltage of 60V to the loads.
Here, FIGS. 25 and 26 are waveform charts respectively illustrating potential and current changes of each component on the circuit of FIG. 24. A waveform WD11 in FIG. 25 shows the potential change of the first input connection T1 with reference to the potential of the second input connection T2; a waveform WD12 in the same figure shows the change of voltage across the resistance unit RU; and a waveform WD13 in the same figure shows the potential change of the second output connection T3 with reference to the potential of the second input connection T2. During the positive part of the waveform WD12, joule losses occur in the resistance unit RU.
A waveform WD14 in FIG. 26 shows the change of current flowing from the AC power supply S to the first input connection T1 when the direction of current flowing from the first input connection T1 toward the AC power supply S is the positive direction. A waveform WD15 in the same figure shows the change of current supplied to the capacitor C11 when the direction of current flowing through the first diode D11 toward the capacitor C11 is the positive direction. A waveform WD16 in the same figure shows the change of current flowing to the Zener diodes ZD11 to ZD13 when the direction of current flowing in the forward direction of the Zener diodes ZD11 to ZD13 is the positive direction.
One of prior-art techniques for stabilizing the terminal voltage of a smoothing capacitor is that described in patent document 1 for use in a full-wave rectifying circuit.
Patent Document 1: Japanese Patent Application Laid-open No. 6-284729